While various processes for producing aliphatic aldehydes have been proposed to date, an optimal means for starting with carboxylic acids or derivatives thereof have not yet been reported.
The most commonly employed of the conventional processes is a process utilizing so-called Rosenmund reduction of a carboxylic acid chloride. This process has a disadvantage of high cost incurred.
Direct reduction of carboxylic acids with molecular hydrogen would be the most advantageous process for the production of aldehydes, but it has been regarded extremely difficult.
As the latest technique, U.S. Pat. No. 4,328,373 proposes a process in which methyl isobutyrate or methyl pivalate is hydrogenated in a gaseous phase in the presence of a yttrium oxide catalyst to obtain a corresponding aldehyde. This process, however, starts with a methyl ester of a carboxylic acid not with a free carboxylic acid. Moreover, the temperature employed is high throughout the reaction, and the yield of a desired aldehyde is low due to side reactions.
The present inventors previously proposed a process in which an aliphatic carboxylic acid is hydrogenated in the presence of zirconium oxide as a catalyst to obtain a corresponding aldehyde as disclosed in JP-A-62-108832 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). As a result of further investigations, however, it turned out that a zirconium oxide catalyst which is prepared by conventional processes needs further improvements to be used in the above-described process, for example, in activity, yield of desired products, working life, etc. In particular, when the starting aliphatic carboxylic acid has a large number of carbon atoms, the yield attained is low.